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Copyright © 2006, ASHRAE
CHAPTER 39
ULTRALOW-TEMPERATURE REFRIGERATION
Autocascade Systems ............................................................... 39.1
Custom-Designed and Field-Erected Systems ......................... 39.2
Single-Refrigerant Systems ...................................................... 39.2
Cascade Systems ...................................................................... 39.3
39
Low-Temperature Materials..................................................... 39.6
Insulation ................................................................................. 39.9
Heat Transfer ........................................................................... 39.9
Secondary Coolants ............................................................... 39.11
LTRALOW-TEMPERATURE refrigeration is defined here Fig. 1 Simple Autocascade Refrigeration System
Uas refrigeration in the temperature range of –50 to –100°C.
What is considered low temperature for an application depends on
the temperature range for that specific application. Low tem-
peratures for air conditioning are around 0°C; for industrial refriger-
ation, –35 to –50°C; and for cryogenics, approaching 0 K.
Applications such as freeze-drying, as well as the pharmaceutical,
chemical, and petroleum industries, use refrigeration in the temper-
ature range designated low in this chapter.
The –50 to –100°C temperature range is treated separately be-
cause design and construction considerations for systems that
operate in this range differ from those encountered in industrial
refrigeration and cryogenics, which bracket it. Designers and
builders of cryogenic facilities are rarely active in the low-temper-
ature refrigeration field. One major type of low-temperature sys-
tem is the packaged type, which often serves such applications as
environment chambers. The other major category is custom-
designed and field-erected systems. Industrial refrigeration prac-
titioners are the group most likely to be responsible for these
systems, but they may deal with low-temperature systems only oc-
casionally; the experience of a single organization does not accu-
mulate rapidly. The objective of this chapter is to bring together
available experience for those whose work does not require daily
contact with low-temperature systems.
The refrigeration cycles presented in this chapter may be used
in both standard packaged and custom-designed systems. Cascade
systems are emphasized, both autocascade (typical of packaged
units) and two-refrigerant cascade (found in custom-engineered
low-temperature systems).
AUTOCASCADE SYSTEMS
An autocascade refrigeration system is a complete, self-contained
refrigeration system in which multiple stages of cascade cooling
effect occur simultaneously by means of vapor/liquid separation and
adiabatic expansion of various refrigerants. Physical and thermody-
namic features, along with a series of counterflow heat exchangers
and an appropriate mixture of refrigerants, allow the system to reach
low temperature.
Autocascade refrigeration systems offer many benefits, such as a
low compression ratio and relatively high volumetric efficiency.
However, system chemistry and heat exchangers are complex, refrig-
erant compositions are sensitive, and compressor displacement is
large.
Operational Characteristics
Components of an autocascade refrigeration system typically
include a vapor compressor, an external air- or water-cooled con-
denser, a mixture of refrigerants with descending boiling points, and
a series of insulated heat exchangers. Figure 1 is a schematic of a
simple system illustrating a single-stage of autocascade.
The preparation of this chapter is assigned to TC 10.4, Ultralow-Tempera-
ture Systems and Cryogenics.
In this system, two refrigerants with significantly different boiling
points are compressed and circulated by one vapor compressor. As-
sume that one refrigerant is R-23 (normal boiling point, –82°C) and
the second refrigerant is R-404a (normal boiling point, –46.7°C).
Assume that ambient temperature is 25°C and that the condenser is
100% efficient.
With properly sized components, this system should be able to
achieve –60°C in the absorber while the compression ratio is main-
tained at 5.1 to 1. As the refrigerant mixture is pumped through the
main condenser and cooled to 25°C at the exit, compressor dis-
charge pressure is maintained at 1524 kPa (gage). At this condition,
virtually all R-404a is condensed at 35°C and then further chilled to
subcooled liquid. Although R-23 molecules are present in both liq-
uid and vapor phases, the R-23 is primarily vapor because of the
large difference in the boiling points of the two refrigerants. A phase
separator at the outlet of the condenser collects the liquid by gravi-
tational effect, and the R-23-rich vapor is removed from the outlet of
the phase separator to the heat exchanger.
At the bottom of the phase separator, an expansion device adia-
batically expands the collected R-404a-rich liquid such that the out-
let of the device produces a low temperature of –19°C at 220 kPa
(gage) (Weng 1995). This cold stream is immediately sent back to
the heat exchanger in a counterflow pattern to condense the R-23-
rich vapor to liquid at –18.5°C and 1524 kPa (gage). The R-23-rich
liquid is then adiabatically expanded by a second expansion device
to –60°C. As it absorbs an appropriate amount of heat in the ab-
sorber, the R-23 mixes with the expanded R-404a and evaporates in
the heat exchanger, providing a cold source for condensing R-23 on
the high-pressure side of the heat exchanger. Leaving the heat ex-
changer at superheated conditions, the vapor mixture then returns to
the suction of the compressor for the next cycle.
Fig. 1 Simple Autocascade Refrigeration System
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39.2 2006 ASHRAE Handbook—Refrigeration (SI)
As can be seen from this simple example, the autocascade effect The design process includes selection of (1) metal for piping and
Fig. 2 Four-Stage Autocascade System
Fig. 2 Four-Stage Autocascade System
 
derives from a short cycle of the refrigerant circuit within the sys-
tem that performs only internal work to condense the lower boiling
point refrigerant.
The concept of the single-stage cycle can be extended to multiple
stages. Figure 2 shows the flow diagram of a four-stage system. The
condensation and subsequent expansion of one refrigerant provides
the cooling necessary to condense the next refrigerant in the heat
exchanger downstream. This process continues until the last refrig-
erant with the lowest boiling point is expanded to achieve extremely
low temperature.
Design Considerations
Compressor Capacity. As can be seen from Figures 1 and 2, a
significant amount of compressor work is used for internal evapo-
rating and condensing of refrigerants. The final gain of the system is
therefore relatively small. Compressor capacity must be enough to
produce an appropriate amount of final refrigerating effect.
Heat Exchanger Sizing. Because there is a significant amount
of refrigerant vapor in each stage of the heat exchanger, the overall
heat transfer coefficients on both the evaporating and condensing
sides are rather small compared to those of pure components at
phase-changing conditions. Therefore, generous heat-transfer area
should be provided for energy exchange between refrigerants on the
high- and low-pressure sides.
Expansion Devices. Each expansion device is sized to provide
sufficient refrigerating effect for the adjacent downstream heat
exchanger.
Compressor Lubrication. General guidelines for lubrication of
refrigeration systems should be adopted.
CUSTOM-DESIGNED AND FIELD-ERECTED 
SYSTEMS
If refrigeration is to maintain a space at a low temperature to store
a modest quantity of product in a chest or cabinet, the packaged low-
temperature system is probably the best choice. Prefabricated walk-
in environmental chambers are alsopractical solutions when they
can accommodate space needs. When the required refrigeration
capacity exceeds that of packaged systems, or when a fluid must be
chilled, a custom-engineered system should be considered.
The refrigeration requirement may be to chill a certain flow rate
of a given fluid from one temperature to another. Part of the design
process is to choose the type of system, which may be a multistage
plant using a single refrigerant or a two-circuit cascade system using
a high-pressure refrigerant for the low-temperature circuit. The
compressor(s) and condenser(s) must be selected, and the evapora-
tor and interstage heat exchanger (in the case of the cascade system)
must be either selected or custom-designed. 
vessels and (2) insulating material and method of application. The
product to be refrigerated may actually pass through the evaporator,
but in many cases a secondary coolant transfers heat from the final
product to the evaporator. Brines and antifreezes that perform sat-
isfactorily at higher temperatures may not be suitable at low tem-
peratures. Compressors are subjected to unusual demands when
operating at low temperatures, and, because they must be lubricated,
oil selection and handling must be addressed.
SINGLE-REFRIGERANT SYSTEMS
Single-refrigerant systems are contrasted with the cascade system,
which consists of two separate but thermally connected refrigerant
circuits, each with a different refrigerant (Stoecker and Jones 1982).
In the industrial refrigeration sector, the traditional refrigerants
have been R-22 and ammonia (R-717). Because R-22 will ulti-
mately be phased out, various hydrofluorocarbon (HFC) refriger-
ants and blends are proposed as replacements. Two that might be
considered are R-507 and R-404a.
Two-Stage Systems
In systems where the evaporator operates below about –20°C,
two-stage or compound systems are widely used. These systems are
explained in Chapter 3 of this volume and in Chapter 1 of the 2005
ASHRAE Handbook—Fundamentals. Advantages of two-stage
compound systems that become particularly prominent when the
evaporator operates at low temperature include
• Improved energy efficiency because of removal of flash gas at the
intermediate pressure and desuperheating of discharge gas from
the low-stage compressor before it enters the high-stage com-
pressor.
• Improved energy efficiency because two-stage compressors are
more efficient operating against discharge-to-suction pressure
ratios that are lower than for a single-stage compressor.
• Avoidance of high discharge temperatures typical of single-stage
compression. This is important in reciprocating compressors but
of less concern with oil-injected screw compressors.
• Possibility of a lower flow rate of liquid refrigerant to the evapo-
rator because the liquid is at the saturation temperature of the
intermediate pressure rather than the condensing pressure, as is
true of single-stage operation.
Refrigerant and Compressor Selection
 The compound, two-stage (or even three-stage) system is an
obvious possibility for low-temperature applications. However, at
very low temperatures, limitations of the refrigerant itself appear:
freezing point, pressure ratios required of the compressors, and
volumetric flow at the suction of the low-stage compressor per unit
Ultralow-Temperature Refrigeration 39.3
 
refrigeration capacity. Table 1 shows some key values for four can-
didate refrigerants, illustrating some of the concerns that arise when
considering refrigerants that are widely applied in industrial refrig-
eration systems. Hydrocarbons (HCs), which are candidates partic-
ularly in the petroleum and petrochemical industry, where the entire
plant is geared toward working with flammable gases, are not
included in Table 1.
The freezing point is not a limitation for the halocarbon refrig-
erants, but ammonia freezes at –77.8°C, so its use must be
restricted to temperatures safely above that temperature.
The pressure ratios the compressors must operate against in
two-stage systems are also important. A condensing temperature of
35°C is assumed, with the intermediate pressure being the geometric
mean of the condensing and evaporating pressures. Many low-
temperature systems may be small enough that a reciprocating com-
pressor would be favorable, but the limiting pressure ratio with
reciprocating compressors is usually about 8, a value chosen to limit
the discharge temperature. An evaporating temperature of –70°C is
about the lowest permissible for systems using reciprocating com-
pressors. For evaporating temperatures lower than –70°C, consider
using a three-stage system. An alternative to the reciprocating com-
pressor is the screw compressor, which operates with lower dis-
charge temperatures because it is oil flooded. The screw compressor
can therefore operate against larger pressure ratios than the recipro-
cating compressor, and is favored in larger systems.
The required volumetric pumping capacity of the compressor
is measured at the compressor suction. This value is an indicator of
the physical size of the compressor; the values become huge at the
–90°C evaporating temperature.
Some conclusions from Table 1 are
• A single-refrigerant, two-stage system can adequately serve a
plant in the higher-temperature portion of the range considered
here, but it becomes impractical in the lower-temperature portion.
• Ammonia, which has many favorable properties for industrial
refrigeration, has little appeal for low-temperature refrigeration
because of its relatively high freezing point and pressure ratios.
Special Multistage Systems
 Special high-efficiency operations to recover volatile com-
pounds such as hydrocarbons use the reverse Brayton cycle. This
consists of one or two conventional compressor refrigeration cycles
with the lowest stage ranging from –60 to –100°C. This final stage
is achieved by using a turbo compressor/expander and enables the
collection of liquefied hydrocarbons (Emhö 1997; Enneking and
Priebe 1993; Jain and Enneking 1995).
CASCADE SYSTEMS
The cascade system (Figure 3) confronts some of the problems
of single-refrigerant systems. It consists of two separate circuits,
each using a refrigerant appropriate for its temperature range. The
two circuits are thermally connected by the cascade condenser,
which is the condenser of the low-temperature circuit and the evap-
orator of the high-temperature circuit. Typical refrigerants for the
Table 1 Low-Temperature Characteristics of Several 
Refrigerants at Three Evaporating Temperatures
Refrige
rant
Freezing 
Point
Pressure Ratio with
Two-Stage System
Volumetric Flow of 
Refrigerant, L/s per kW
Evaporating Temp. Evaporating Temp.
–50°C –70°C –90°C –50°C –70°C –90°C
R-22 –160°C 4.6 8.14 17.9 1.57 4.81 19.8
R-507 ≤100°C 4.4 7.8 16.1 1.29 4.08 17.5
R-717 –77.8°C 5.75 11.1 25.4 2.05 7.24 —
R-404a ≤100°C 4.36 7.58 15.2 1.34 4.13 16.75
high-temperature circuit include R-22, ammonia, R-507, and R-
404a. For the low-temperature circuit, a high-pressure refrigerant
with a high vapor density (even at low temperatures) is chosen. For
many years, R-503, an azeotropic mixture of R-13 and R-23, was a
popular choice, but R-503 is no longer available because R-13 is an
ozone-depleting chlorofluorocarbon (CFC). R-23 could be and has
been used alone, but R-508b, an azeotrope of R-23 and R-116, has
superior properties, as discussed in the section on Refrigerants for
Low-Temperature Circuit.
The cascade system has some of the thermal advantages of two-
stage, single-refrigerant systems: it approximates flash gas removal
and allows each compressor to take a share of the total pressure
ratio between the low-temperature evaporator and the condenser.
The cascade system has the thermal disadvantage of needing to
provide an additional temperature lift in the cascade condenser
because the condensing temperature of the low-temperature refrig-
erant is higher than the evaporating temperature of the high-tem-
perature refrigerant. There is anoptimum operating temperature of
the cascade condenser for minimum total power requirement, just
as there is an optimum intermediate pressure in two-stage, single-
refrigerant systems.
Figure 3 shows a fade-out vessel, which limits pressure in the
low-temperature circuit when the system shuts down. At room
temperature, the pressure of R-23 or R-508b in the system would
exceed 4000 kPa if liquid were present. The entire low-tempera-
ture system must be able to accommodate this pressure. The pro-
cess occurring in the fade-out vessel is at constant volume, as
shown on the pressure-enthalpy diagram of Figure 4. When the
Fig. 3 Simple Cascade System
Fig. 3 Simple Cascade System
Fig. 4 Simple Cascade Pressure-Enthalpy Diagram
Fig. 4 Simple Cascade Pressure-Enthalpy Diagram
39.4 2006 ASHRAE Handbook—Refrigeration (SI)
system operates at low temperature, the refrigerant in the system
is a mixture of liquid and vapor, indicated by point A. When the
system shuts down, the refrigerant begins to warm and follows the
constant-volume line, with pressure increasing according to the
saturation curve. When the saturated vapor line is reached at point
B, further increases in temperature result in only slight increases
in pressure because the refrigerant is superheated vapor.
Larger cascade systems are field engineered, but packaged sys-
tems are also available. Figure 5 shows a two-stage system with the
necessary auxiliary equipment. Figure 6 shows a three-stage cas-
cade system. 
Refrigerantsfor Low-Temperature Circuit
R-503 is no longer available for general use. R-23 can be used,
but R-508b, an azeotropic mixture of non-ozone-depleting R-23 and
Fig. 5 Two-Stage Cascade System
Fig. 5 Two-Stage Cascade System
 
Fig. 6 Three-Stage Cascade System
Fig. 6 Three-Stage Cascade System
Ultralow-Temperature Refrigeration 39.5
 
R-116, is superior. It is nonflammable and has zero ozone depletion
potential (ODP). Table 2 lists some properties of R-508b.
R-508b offers excellent operating characteristics compared to
R-503 and R-13. Capacity and efficiency values are nearly equiva-
lent to R-503’s and superior to R-13’s. The compressor discharge
temperature is lower than for R-23; lower discharge temperatures
may equate to longer compressor life and better lubricant stability.
The estimated operating values of a cascade system running with
R-508b are shown in Table 3. Performance parameters of R-503,
R-13, and R-23 are shown for comparison.
Table 4 shows calculated data for R-23 and R-508b for two
operating ranges. The volumetric efficiency is 100%. Actual com-
pressor performance varies with pressure ratios and yields lower ca-
pacity and efficiencies and higher discharge temperatures and flow
requirements than shown.
Compressor Lubrication
When selecting a lubricant to use with R-508b in an existing low-
temperature system, consider (1) refrigerant/lubricant miscibility,
(2) chemical stability, (3) materials compatibility, and (4) refrigera-
tion system design. Original equipment manufacturers and compres-
sor suppliers should be consulted.
Using additives to enhance system performance is well estab-
lished in the low-temperature industry, and may be applied to
R-508b. The miscibility of R-508b with certain polyol esters
(POEs) is slightly better than the limited miscibility of R-13 and
Table 2 Properties of R-508b
Boiling point (101.325 kPa) –88°C
Critical temperature 13.7°C
Critical pressure 3935 kPa
Latent heat of vaporization at boiling point 168.4 kJ/kg
Ozone depletion potential (R-12 = 1) 0
Flammability Nonflammable
Exposure limit (8 and 12 h)* 1000 ppm
*The exposure limit is a calculated limit determined from the DuPont airborne expo-
sure limit (AEL) of the individual components. The AEL is the maximum amount to
which nearly all workers can be repeatedly exposed during a working lifetime without
adverse effects.
Table 3 Theoretical Performance of a Cascade System Using 
R-13, R-503, R-23, or R-508b
R-503 R-13 R-23 R-508b
Capacity (R-503 = 100) 100 71 74 98
Efficiency (R-503 = 100) 100 105 95 103
Discharge pressure, kPa 999 717 848 1013
Suction pressure, kPa 110 83 90 110
Discharge temperature, °C 107 92 138 87
Note: Operating conditions are –84.4°C evaporator, –35°C condenser; 5.6 K subcool-
ing; –17.8°C suction temperature; 70% isentropic compression efficiency, 4% volu-
metric clearance.
Table 4 Theoretical Compressor Performance Data for Two 
Different Evaporating Temperatures
Evaporating
Temperature,
°C Refrigerant
Pressure
Ratio
Discharge
Temperature,
°C
Volumetric 
Flow,
L/s per kW
–80 R-23 7.49 58 1.10
R-508b 6.51 32 0.866
–100 R-23 26.88 72 3.85
R-508b 21.88 39 2.94
Basis: –35°C condensing temperature; compressor efficiency of 70%; volumetric effi-
ciency of 100%; 10 K subcooling, and 50 K suction superheat.
R-503 with mineral oil and alkylbenzene, which helps oil circu-
lation at the low evaporator temperatures. Even with increased mis-
cibility, additives may enhance performance. Some POE oils
designed for use in very-low-temperature systems have been used
successfully with R-508b in equipment retrofits. Consult compres-
sor manufacturers and suppliers before a final decision on lubricants
and any additives.
Compressors
Larger cascade systems typically use standard, positive-
displacement compressors in the dual refrigeration system. The
evaporator of the higher-temperature refrigerant system serves as
the condenser for the lower-temperature one. This allows rather nor-
mal application of the compressors on both systems in relation to the
pressures, compression ratios, and oil and discharge temperatures
within the compressors. However, several very important items
must be considered for both the high and low sides of the cascade
system to avoid operational problems. Commercially available
compressors must be analyzed for both sides of the cascade to deter-
mine the best combination for a suitable intermediate high-side
evaporator/low-side condenser and minimum (or economical) sys-
tem power usage.
High-Temperature Circuit. The higher-temperature system is
generally a single- or two-stage system using a commercial refrig-
erant (R-134a, R-22, R-404a, or R-717); evaporating temperature is
approximately –23 to –45°C, and condensing is at normal ambient
conditions. Commercially available reciprocating and screw com-
pressors are suitable. If compressor evaporating temperature is
below –45°C, a suction-line heat exchanger is needed to superheat
compressor suction gas to at least –43°C to avoid the metal brittle-
ness associated with lower temperatures at the compressor suction
valve and body. Suction piping materials and the evaporator/
condenser (evaporator side) must also be suitable for these low tem-
peratures per American Society of Mechanical Engineers (ASME)
code requirements.
Lubricant for the higher-temperature system must be compatible
with the refrigerant and suitable for the type of system, consider-
ing oil carryover and return from the evaporator and the low-
temperature conditions within the evaporator.
Low-Temperature Circuit. The compressor may also be a stan-
dard refrigeration compressor, if the compressor suction gas is super-
heated to at least –43°C to avoid low-temperature metal brittleness.
Operation is typically well within standard pressure, oil temperature,
and discharge temperature limits. The refrigerant is usually R-23 or
R-508b. Because temperatures in the low side are below –45°C, all
piping, valves, and vessels must be of materials that comply with
ASME codes for these temperatures.
It may be difficult to obtain compressor rating data for low-
temperature applications with these refrigerants because few
actual test data are available, and the manufacturer may be reluc-
tant to be specific. Therefore, the low side should not be designed
too close to the required specification. Good practice is to calcu-
late the actual volumetric flow rate to be handled by the compres-
sor (at the expected superheat) to be certain that it canperform as
required.
Capacity loss from high superheat is more than recouped (for a
net capacity gain) from the liquid subcooling obtained by the suc-
tion-line heat exchanger. In rare cases, it may be necessary to inject
a small quantity of hot gas into the suction to ensure maximum suc-
tion temperature.
The lubricant selected for the low side must be compatible with
the specific refrigerant used and also suitable for the low tempera-
tures expected in the evaporator. It is important that adequate coa-
lescing oil separation (5 ppm) is provided to minimize oil carryover
from compressor to evaporator.
In direct-expansion evaporators, any oil is forced through the
tubes, but speed in the return lines must be high enough to keep
39.6 2006 ASHRAE Handbook—Refrigeration (SI)
 
this small amount of oil moving back to the compressor. If the sys-
tem has capacity control, then multiple suction risers or alternative
design procedures may be required to prevent oil logging in the
evaporator and ensure oil return. At these ultralow temperatures, it
is imperative to select a lubricant that remains fluid and does not
plate out on the evaporator surfaces, where it can foul heat transfer.
Choice of Metal for Piping and Vessels
The usual construction metal for use with thermal fluids is car-
bon steel. However, carbon steel should not be used below –29°C
because of its loss of ductility. Consider using 304 or 316 stainless
steel because of their good low-temperature ductility. Another alter-
native is to use carbon steel that has been manufactured specifically
to retain good ductility at low temperatures (Dow Corning USA
1993). For example,
Carbon steel Down to 29°C
SA - 333 - GR1 –29 to –46°C
SA - 333 - GR7 –46 to –73°C
SA - 333 - GR3 –59 to –101°C
SA - 333 - GR6 To 46°C
LOW-TEMPERATURE MATERIALS
Choosing material for a specific low-temperature use is often a
compromise involving several factors:
• Cost
• Stress level at which the product will operate
• Manufacturing alternatives
• Operating temperature
• Ability to weld and stress-relieve welded joints
• Possibility of excessive moisture and corrosion
• Thermal expansion and modulus of elasticity characteristics for
bolting and connection of dissimilar materials
• Thermal conductivity and resistance to thermal shock
Effect of Low Temperature on Materials. When the piping and
vessels are to contain refrigerant at low temperature, special mate-
rials must usually be chosen because of the effect of low tem-
perature on material properties. Chemical interactions between the
refrigerant and containment material must also be considered.
Mechanical and physical properties, fabricability, and availability
are some of the important factors to consider. Few generalizations
can be made, except that decreased temperature increases hardness,
strength, and modulus of elasticity. The effect of low temperatures
on ductility and toughness can vary considerably between materials.
With a decrease in temperature, some metals show increased ductil-
ity; others show increased some limiting low temperature, followed
by a decrease at lower temperatures. Still other metals decrease in
toughness and ductility as temperature decreased below room tem-
perature.
The effect of temperature reduction on polymers depends on the
type of polymer. Thermoplastic polymers, which soften when
heated above their glass transition temperature Tg, become progres-
sively stiffer and finally brittle at low temperatures. Thermosetting
plastics, which are highly cross-linked and do not soften when
heated, are brittle at both ambient and lower temperatures. Elas-
tomers (rubbers) are lightly cross-linked and stiffen like thermo-
plastics as the temperature is lowered, becoming fully brittle at very
low temperatures.
Although polymers become brittle and may crack at low temper-
atures, their unique combination of properties (excellent thermal
and electrical insulation capability, low density, low heat capacity,
and nonmagnetic character) make them attractive for a variety of
lower-temperature applications. At extremely low temperatures, all
plastic materials are very brittle and have low thermal conductivity
and low strength relative to metals and composites, so selection and
use must be carefully evaluated.
Fiber composite materials have gained widespread use at low
temperatures, despite their incorporation of components that are
often by themselves brittle. A factor that must be considered in the
use of composites is the possibility of anisotropic behavior, in
which they exhibit properties with different values when measured
along axes in different directions. Composites with aligned fibers
are highly anisotropic.
Metals
The relation of tensile strength to temperature for common struc-
tural metals at low temperatures is shown in Figure 7 (Askeland
1994). The slopes of the curves indicate that the increase in strength
with decrease in temperature varies. However, tensile strength is not
the best criterion for determining the suitability of a material for
low-temperature service, because most failures result from a loss of
ductility.
Lower temperatures can have a dramatic effect on the ductility
of metal; the effect depends to a large extent on crystal structure.
Metals and alloys that are face-centered cubic (FCC) and ductile at
ambient temperatures remain ductile at low temperatures; this cate-
gory includes aluminum, copper, copper-nickel alloys, nickel, and
austenitic stainless steels. Metals and alloys that are body-centered
cubic (BCC), such as pure iron, carbon steel, and many alloy steels,
become brittle at low temperatures. Many BCC metals and alloys
exhibit a ductile-to-brittle transition at lower temperatures (see 1020
steel in Figure 8). This loss of ductility comes from a decrease in the
number of operating slip systems, which accommodate dislocation
motion. Hexagonal close-packed (HCP) metals and alloys occupy
an intermediate place between FCC and BCC materials and may
Fig. 7 Tensile Strength Versus Temperature of
Several Metals
Fig. 7 Tensile Strength Versus Temperature of 
Several Metals
Fig. 8 Tensile Elongation Versus Temperature of
Several Metals
Fig. 8 Tensile Elongation Versus Temperature of 
Several Metals
Ultralow-Temperature Refrigeration 39.7
 
Table 5 Several Mechanical Properties of Aluminum Alloys at 196°C
Aluminum Alloy
Modulus of Elasticity, 
GPa
Yield Strength,
MPa
Ultimate Tensile 
Strength, GPa
Percent Elongation
at Failure, %
Plane Strain Fracture 
Toughness, MPa ·m1/2
1100-0 78 50 190 — —
2219-T851 85 440 568 14 45
5083-0 80 158 434 32 62
6061-T651 77 337 402 23 42
remain ductile or become brittle at low temperatures. Zinc be-
comes brittle, whereas pure titanium (Ti) and many Ti alloys
remain ductile.
Ductility values obtained from the static tensile test may give
some clue to ductility loss, but the notched-bar impact test gives a
better indication of how the material performs under dynamic load-
ing and how it reacts to complex multidirectional stress. Figure 8
shows ductility, as measured by percent elongation in the tensile
test, in relation to temperature for several metals. As temperature
drops, the curves for copper and aluminum show an increase in
ductility, while AISI 304 stainless steel and Ti-6%Al-4%V show a
decrease.
Aluminum alloys are used extensively for low-temperature
structural applications because of cost, weldability, and toughness.
Although their strength is considered modest to intermediate, they
remain ductile at lower temperatures. Typical mechanical prop-
erties at –196°C are listed in Table 5. Property values between am-
bient (21°C) and –196°C are intermediate between those at these
temperatures.
Aluminum 1100 (relatively pure at 99% Al) has a low yield
strength but is highly ductile and has a high thermal conductivity. It
is used in nonstructural applications such as thermal radiation
shields. For structural purposes, alloys 5083, 5086, 5454, and 5456
are often used. Alloys such as 5083 have a comparativelyhigh
strength when annealed (0) and can be readily welded with little loss
of strength in the heat-affected zone; post-welding heat treatments
are not necessary. These alloys are used in the storage and transpor-
tation areas. Alloy 3003 is widely used for plate-fin heat exchangers
because it is easily brazed with an Al-7%Si filler metal. Aluminum-
magnesium alloys (6000 series) are used as extrusions and forgings
for such components as pipes, tubes, fittings, and valve bodies.
Copper alloys are rarely used for structural applications because
of joining difficulties. Copper and its alloys behave similarly to alu-
minum alloys as temperature decreases. Strength is typically in-
versely proportional to impact resistance; high-strength alloys have
low impact resistance. Silver soldering and vacuum brazing are the
most successful methods for joining copper. Brass is useful for
small components and is easily machined.
Nickel and nickel alloys do not exhibit a ductile-to-brittle tran-
sition as temperature decreases and can be welded successfully, but
their high cost limits use. High-strength alloys can be used at very
low temperatures.
Iron-based alloys that are body-centered cubic usually exhibit a
ductile-to-brittle transition as the temperature decreases. The BCC
phase of iron is ferromagnetic and easily identified because it is
attracted to a magnet. Extreme brittleness is often observed at lower
temperatures. Thus, BCC metals and alloys are not normally used
for structural applications at lower temperatures. Notable excep-
tions are iron alloys with a high nickel content.
Nickel and manganese are added to iron to stabilize the austenitic
phase (FCC), promoting low-temperature ductility. Depending on
the amount of Ni or Mn added, a great deal of low-temperature
toughness can be developed. Two notable high-nickel alloys for use
below ambient temperature include 9% nickel steel and austenitic
36% Ni iron alloy. The 9% alloy retains good ductility down to
100 K (–173°C). Below 100 K, ductility decreases slightly, but a
clear ductile-to-brittle transition does not occur. Iron containing
36% Ni has the unusual feature of nearly zero thermal contraction
during cooling from room temperature to near absolute zero. It is
therefore an attractive metal for subambient use where the thermal
stress associated with differential thermal contraction is to be
avoided. Unfortunately, this alloy is quite expensive and therefore
sees limited use.
Lesser amounts of Ni can be added to Fe to lower cost and depress
the ductile-to-brittle transition temperature. Iron with 5% Ni can be
used down to 150 K (–123°C), and Fe with 3.5% Ni remains ductile
to 170 K (–102°C). High-nickel steels are usually heat treated before
use by water quenching from 800°C, followed by tempering at
580°C. The 580°C heat treatment tempers martensite formed during
quenching and produces 10 to 15% stable austenite, which is respon-
sible for the improved toughness of the product.
The austenitic stainless steels (300 series) are widely used for
low-temperature applications. Many retain high ductility down to
4 K (–269°C) and below. Their attractiveness is based on good
strength, stiffness, toughness, and corrosion resistance, but cost is
high compared to that of Fe-C alloys. A stress relief heat treatment
is generally not required after welding, and impact strengths vary
only slightly with decreasing temperature. A popular, readily avail-
able steel with moderate strength for low-temperature service is
AISI type 304, with the low-carbon grade preferred. Where higher
strengths are needed and welding can be avoided, strain-hardened or
high-nitrogen grades are available. Castable austenitic steels are
also available; a well-known example (14-17%Cr, 18-22%Ni, 1.75-
2.75%Mo, 0.5%Si max, and 0.05%C max) retains excellent ductil-
ity and strength to extremely low temperatures.
Titanium alloys have high strength, low density, and poor ther-
mal conductivity. Two alloys often used at low temperatures are Ti-
5%Al-2.5%Sn and Ti-6%Al-4%V. The Ti-6-4 alloy has the higher
yield strength, but loses ductility below about 80 K (–193°C). The
low-temperature properties are dramatically affected by oxygen,
carbon, and nitrogen content. Higher levels of these interstitial ele-
ments increase strength but decrease ductility. Extra-low interstitial
(ELI) grades containing about half the normal levels are usually
specified for low-temperature applications. Both Ti-6-4 and Ti-5-
2.5 are easily welded but expensive and difficult to form. They are
used where a high strength-to-mass or strength-to-thermal conduc-
tivity ratio is attractive. Titanium alloys are not recommended for
applications where an oxidation hazard exists.
Thermoplastic Polymers
Reducing the temperature of thermoplastic polymers restricts
molecular motion (bond rotations and molecules sliding past one
another), so that the material becomes less deformable. Behavior
normally changes rapidly over a narrow temperature range, begin-
ning at the material’s glass transition temperature Tg.
Figure 9 shows the general mechanical response of linear
amorphous thermoplastics to temperature. At or above the melting
temperature Tm, bonding between polymer chains is weak, the
material flows easily, and the modulus of elasticity is nearly zero.
Just below Tm, the polymer becomes rubbery; with applied stress,
the material deforms by elastic and plastic strain. The combination
of these deformations is related to the applied stress by the shear
modulus. At still lower temperatures, the polymer becomes stiffer,
exhibiting “leathery” behavior and a higher stress at failure. Many
commercially available polymers (e.g., polyethylene) are used in
this condition. Tg is at the transition between the leathery and
39.8 2006 ASHRAE Handbook—Refrigeration (SI)
 
glassy regions, and is usually 0.5 to 0.75 times the absolute melt-
ing temperature Tm. Table 6 lists Tg and Tm values for common
polymers. In the glassy state, at temperatures below Tg, the poly-
mer is hard, brittle, and glass-like. Although polymers in the glassy
region have poor ductility and formability, they are strong, stiff,
and creep-resistant.
For thermoplastic polymers, the temperature at which stress is
applied and the rate of stress application are interdependent based
on time/temperature superposition. This relationship allows differ-
ent types of tests, such as creep or stress relaxation, to be related
through a single curve that describes the viscoelastic response of the
material to time and temperature. Applying stress more rapidly has
an effect equivalent to applying stress at lower temperatures. Figure
10 shows tensile strength versus temperature for plastic and poly-
mer composites.
Thermoplastic polymers such as polyethylene and polyvinyl chlo-
ride (PVC) may be used for plastic films and wire insulation but are
not generally suitable for structural applications because of their brit-
tleness at temperatures below Tg. The only known polymer that ex-
hibits appreciable ductility at temperatures substantially below Tg is
polytetrafluoroethylene (PTFE). Because of their large thermal
Table 6 Approximate Melting and Glass Transition 
Temperatures for Common Polymers
Polymer
Melting
Temperature Tm
Glass Transition 
Temperature Tg
K °C K °C
Addition polymers
Low-density polyethylene 390 115 155 –120
High-density polyethylene 415 140 155 –120
Polyvinyl chloride — — 365 90
Polypropylene 445 170 260 –15
Polystyrene — — 380 105
Polytetrafluoroethylene 605 330 — —
Polymethyl methacrylate 
(acrylic)
— — 370 95
Condensation polymers
6-6 Nylon 540 265 320 50
Polycarbonate — — 420 145
Polyester 530 255 350 75
Elastomers
Silicone — — 150 –125
Polybutadiene 395 115 185 –90
Polychloroprene 355 80 220 –50
Polyisoprene 305 30 200 –70
Source: Askeland (1994). Derived from Table 15-2, p. 482.
Fig. 9 Shear Modulus Versus Normalized Temperature (T/Tg)
for Thermoplastic Polymers
Fig. 9 Shear Modulus Versus Normalized Temperature 
(T/Tg) for ThermoplasticPolymers
contraction coefficients, thermoplastic polymers should not be re-
strained during cooldown. Large masses should be cooled slowly to
ensure uniform thermal contraction; the coefficient of thermal con-
traction decreases with temperature. Contractions of 1 to 2% in cool-
ing from ambient to 196°C are common. For instance, nylon, PTFE,
and polyethylene contract 1.3, 1.9, and 2.3%, respectively. These val-
ues are large compared to those for metals, which contract 0.2 to 0.3%
over the same temperature range.
Thermosetting Plastics
Thermosetting plastics such as epoxy are relatively unaffected
by changes in temperature. As a class, they are brittle and gener-
ally used in compression and not tension. Care must be taken in
changing their temperature to avoid thermally induced stress,
which could lead to cracking. Adding particulate fillers such as sil-
ica (SiO2) to thermosetting resins can increase elastic modulus and
decrease strength. The main reasons for adding fillers are to reduce
the coefficient of thermal expansion and to improve thermal con-
ductivity. Filled thermosetting resins such as epoxy and polyester
can be made to have coefficients of thermal expansion that closely
match those of metal; they may be used as insulation and spacers
but are not generally used for load-bearing structural applications.
Fiber Composites
Nonmetallic filamentary reinforced composites have gained
wide acceptance for low-temperature structural applications be-
cause they have good strength, low density, and low thermal con-
ductivity. Nonmetallic insulating composites are usually formed
by laminating together layers of fibrous materials in a liquid ther-
mosetting resin such as polyester or epoxy. The fibers are often but
not necessarily continuous; they can take the form of bundles,
mats, yarns, or woven fabrics. The most frequently used fiber ma-
terials include glass, aramid, and carbon. Reinforcing fibers add
considerable mechanical strength to otherwise brittle matrix mate-
rial and can lower the thermal expansion coefficient to a value
comparable to that of metals. High figures of merit (ratio of ther-
mal conductivity to elastic modulus or strength) can reduce refrig-
eration costs substantially from those obtained with fully metallic
configurations. Nonmetallic composites can be used for tanks,
tubes, struts, straps, and overlays in low-temperature refrigeration
systems. These materials perform well in high-loading environ-
ments and under cyclic stress; they do not degrade chemically at
low temperatures.
Different combinations of fiber materials, matrices, loading
fractions, and orientations yield a range of properties. Material
properties are often anisotropic, with maximum properties in the
fiber direction. Composites fail because of cracking in the matrix
Fig. 10 Tensile Strength Versus Temperature of Plastics and
Polymer Matrix Laminates
Fig. 10 Tensile Strength Versus Temperature of Plastics and 
Polymer Matrix Laminates
Ultralow-Temperature Refrigeration 39.9
 
layer perpendicular to the direction of stress. Cracking may prop-
agate along the fibers but does not generally lead to debonding.
Maximum elongations at failure for glass-reinforced composites
are usually 2 to 5%; the material is generally elastic all the way to
failure.
A major advantage of using glass fibers with a thermosetting
binder matrix is the ability to match thermal contraction of the
composite to that of most metals. Aramid fibers produce lami-
nates with lower density but higher cost. With carbon fibers, it is
possible to produce components that show virtually zero contrac-
tion on cooling.
Typical tensile mechanical property data for glass-reinforced
laminates are given in Table 7. Under compressive loading, strength
and modulus values are generally 60 to 70% of those for tensile
loading because of matrix shrinkage away from fibers and micro-
buckling of fibers.
Adhesives
Adhesives for bonding composite materials to themselves or to
other materials include epoxy resins, polyurethanes, polyimides,
and polyheterocyclic resins. Epoxy resins, modified epoxy resins
(with nylon or polyamide), and polyurethanes apparently give the
best overall low-temperature performance. The joint must be prop-
erly designed to account for the different thermal contractions of the
components. It is best to have adhesives operate under compressive
loads. Before bonding, surfaces to be joined should be free of con-
tamination, have uniform fine-scale roughness, and preferably be
chemically cleaned and etched. An even bond gap thickness of 0.1
to 0.2 mm is usually best.
INSULATION
Refrigerated pipe insulation, by necessity, has become an engi-
neered element of the refrigeration system. The complexity and cost
of this element now rival that of the piping system, particularly for
ultralow-temperature systems.
Some factory-assembled, close-coupled systems that operate
intermittently can function with a relatively simple installation of
flexible sponge/foam rubber pipe insulation. Larger systems that
operate continuously require much more investment in design and
installation. Higher-technology materials and techniques, which are
sometimes waived (at risk of invested capital) for systems operating
at warmer temperatures, are critical for low-temperature operation.
Also, the nature of the application does not usually allow shutdown
for repair.
Pipe insulation systems are distinctly different from cold-room
construction. Cold-room construction vapor leaks can be neutral if
they reach equilibrium with the dehumidification effect of the
refrigeration unit. Moisture entering the pipe insulation can only
accumulate and form ice, destroying the insulation system. At these
Table 7 Tensile Properties of Unidirectional
Fiber-Reinforced Composites
Composite
Test Tempera-
ture, °C
Tensile Strength,
MPa
Tensile Modulus,
GPa
E Glass (50%)
Longitudinal 22 1050 41
–196 1340 45
Transverse 22 9 11
–196 8 12
Aramid fibers (63%)
Longitudinal 25 1130 71
–196 1150 99
Transverse 25 4.2 2.5
–196 3.6 3.6
Source: Hands (1996). Table 11.3.
low temperatures, it is proper to have redundant vapor retarders
(e.g., reinforced mastic plus membrane plus sealed jacket). Insula-
tion should be multilayer to allow expansion and contraction, with
inner plies allowed to slide and the outer ply joint sealed. Sealants
are placed in the warmest location because they may not function
properly at the lower temperature of inner plies. Insulation should
be thick enough to prevent condensation (above dew point) at the
outside surface.
The main components of a low-temperature refrigerated pipe
insulation system are shown in Table 8. See Chapter 33 for more
information on insulation systems for refrigerant piping.
HEAT TRANSFER
The heat transfer coefficients of boiling and condensing refriger-
ant and the convection heat transfer coefficients of secondary cool-
ants are the most critical heat transfer issues in low-temperature
refrigeration. In a cascade system, for example, the heat transfer
coefficients in the high-temperature circuit are typical of other refrig-
eration applications at those temperatures. In the low-temperature
circuit, however, the lower temperatures appreciably alter the refrig-
erant properties and therefore the boiling and condensing coeffi-
cients.
Table 8 Components of a Low-Temperature Refrigerated 
Pipe Insulation System
Insulation 
System 
Component Primary Roles Secondary Roles Typical Materials
Insulation Efficiently insulate 
the pipe
Provide external 
hanger support
Limit water 
movement 
toward pipe
Reduce rate of 
moisture/vapor 
transfer toward 
pipe
Protect vapor 
retarder from 
external damage
Polyurethane-
modified 
polyisocyanurate 
foams
Extruded 
polystyrene foams
Cellular glass
Elastomeric 
joint 
sealant
Limit liquid water 
movement 
through 
insulation 
cracks
Synthetic rubbers
Resins
Reduce rate of 
moisture/vapor 
transfer toward 
pipe
Vapor 
retarder
Severely limit 
moisture transfer 
toward pipeMastic/fabric/mastic
Laminated 
membranes
Eliminate liquid 
water movement 
toward pipe
Protective 
jacket
Protect vapor 
retarder from 
external damage
Reduce moisture/ 
vapor transfer 
toward pipe
Aluminum
Stainless steel
PVC
Limit water 
movement toward 
pipe
Protective 
jacket joint 
sealant
Prevent liquid 
water movement 
through gaps in 
protective jacket
Limit rate of 
moisture/vapor 
transfer toward 
pipe
Vapor stops Isolate damage 
caused by 
moisture 
penetration
Mastic/fabric/
mastic
39.10 2006 ASHRAE Handbook—Refrigeration (SI)
 
The expected changes in properties with a decrease in tempera-
ture are as follows. As temperature drops,
• Density of liquid increases
• Specific volume of vapor increases
• Enthalpy of evaporation increases
• Specific heat of liquid decreases
• Specific heat of vapor decreases
• Viscosity of liquid increases
• Viscosity of vapor decreases
• Thermal conductivity of liquid increases
• Thermal conductivity of vapor decreases
In general, increases in liquid density, enthalpy of evaporation,
specific heats of liquid and vapor, and thermal conductivity of liq-
uid and vapor cause an increase in the boiling and condensing
heat transfer coefficients. Increases in specific volume of vapor
and viscosities of liquid and vapor decrease these heat transfer
coefficients.
Data from laboratory tests or even field observations are scarce
for low-temperature heat transfer coefficients. However, heat trans-
fer principles indicate that, in most cases, lowering the temperature
level at which heat transfer occurs reduces the coefficient. The low-
temperature circuit in a custom-engineered cascade system encoun-
ters lower-temperature boiling and condensation than are typical of
industrial refrigeration. In some installations, refrigerant boiling is
within the tubes; in others, it is outside the tubes. Similarly, the
designer must decide whether condensation at the cascade con-
denser occurs inside or outside the tubes.
Some relative values based on correlations in Chapter 4 of the
2005 ASHRAE Handbook—Fundamentals may help the designer
determine which situations call for conservative sizing of heat
exchangers. The values in the following subsections are based on
changes in properties of R-22 because data for this refrigerant are
available down to very low temperatures. Other halocarbon refrig-
erants used in the low-temperature circuit of the cascade system are
likely to behave similarly. Predictions are complicated by the fact
that, in a process inside tubes, the coefficient changes constantly as
the refrigerant passes through the circuit. For both boiling and con-
densing, temperature has a more moderate effect when the process
occurs outside the tubes than when it occurs inside the tubes.
A critical factor in the correlations for boiling or condensing
inside the tubes is the mass velocity G in g/(s·m2). The relative
values given in the following subsections are based on keeping G
in the tubes constant. The result is that G drops significantly
because the specific volume of vapor experiences the greatest
relative change of all the properties. As the vapor becomes less
dense, the linear velocity can be increased and still maintain a
tolerable pressure drop of the refrigerant through the tubes. So G
would not drop to the extent used in the comparison below, and
the reductions shown for tube-side boiling and condensing would
not be as severe as shown.
Table 9 Overview of Some Secondary Coolants
Coolant
Flash 
Point, °C
Freezing
Point, °C
Boiling
Point, °C
Temperature at 
WhichViscosity
> 10 mm2/s
Polydimethyl-
siloxane
46.7 –111.1 175 –60
d-Limonene 46.1 96.7 154.4 –80
Diethylbenzene* 58 ≤84 181 –80
58 –75 181 –70
Hydrofluoroether not flamm. –130 60 –30
Ethanol 12 –117 78 –60
Methanol 11 –98 64 –90
*Two proprietary versions containing different additives.
Table 10 Refrigerant Properties of Some Low-Temperature 
Secondary Coolants
Temperature,
°C
Viscosity,
mPa·s
Density,
kg/m3
Heat Capacity,
kJ/(kg· K)
Thermal
Conductivity, 
W/(m·K)
Polydimethylsiloxanea
–100 78.6 978 1.52 0.1340
–90 33.7 968 1.54 0.1323
–80 20.1 958 1.56 0.1305
–70 13.3 948 1.58 0.1288
–60 9.4 937 1.60 0.1269
–50 6.4 927 1.62 0.1250
d-Limoneneb
–80 1.8 929.6 — 0.139
–70 1.7 921.0 — 0.137
–60 1.6 912.2 — 0.135
–50 1.5 903.5 1.39 0.133
Diethylbenzenea, c
–90 Below Freezing Point
–80 10.0 933.6 1.570 0.1497
–70 7.11 926.7 1.594 0.1475
–60 5.12 920.0 1.615 0.1454
–50 3.78 913.0 1.636 0.1435
Hydrofluoroether d
–100 21.226 1814 0.933 0.093
–90 10.801 1788 0.954 0.091
–80 6.412 1762 0.975 0.089
–70 4.235 1737 0.992 0.087
–60 3.017 1711 1.013 0.085
–50 2.268 1686 1.033 0.083
Ethanole
–100 47.1 717.0 1.884 0.199
–90 28.3 726.0 1.918 0.198
–80 18.1 735.1 1.943 0.197
–70 12.4 744.1 1.964 0.195
–60 8.7 753.1 1.985 0.194
–50 6.4 762.2 2.011 0.192
Methanole
–100 16.1 720 2.178 0.224
–90 8.8 729 2.203 0.223
–80 5.7 738 2.228 0.222
–70 40.2 747 2.253 0.221
–60 2.98 756 2.278 0.220
–50 22.6 765 2.303 0.219
Acetone
–94 Freezing Point
–90 — — 1.996 0.150
–80 1.19 — 2.042 0.148
–70 0.89 — 2.008 0.146
–60 0.75 — 2.021 0.145
–50 0.75 — 2.029 0.143
20 — 791 — —
Sources: 
aDow Corning USA (1993)
bFlorida Chemical Co. (1994)
cTherminol LT (1992)
d3M Company (1996)
eRaznjevic (1997)
Ultralow-Temperature Refrigeration 39.11
 
Condensation Outside Tubes. Based on Nusselt’s film conden-
sation theory, the condensing coefficient at 20°C, a temperature that
could be encountered in a cascade condenser, would actually be
17% higher than the condensing coefficient in a typical condenser at
30°C because of higher latent heat, liquid density, and thermal con-
ductivity. The penalizing influence of the increase in specific vol-
ume of vapor is not present because this term does not appear in the
Nusselt equation.
Condensation Inside Tubes. Using the correlation of Ackers
and Rosson (Table 3, Chapter 4 of the 2001 ASHRAE Handbook—
Fundamentals) with a constant velocity and thus decreasing the
value of G by one-fifth, the condensation coefficient at 20°C is one-
fourth that at 30°C.
Boiling Inside Tubes. Using the correlation of Pierre [Equation
(1) in Table 2, Chapter 4 of the 2001 ASHRAE Handbook—Funda-
mentals] and maintaining a constant velocity, when the temperature
drops to 70°C, the boiling coefficient drops to 46% of the value at
20°C.
Boiling Outside Tubes. In a flooded evaporator with refrigerant
boiling outside the tubes, the heat-transfer coefficient also drops as
the temperature drops. Once again, the high specific volume of
vapor is a major factor, restricting the ability of liquid to be in con-
tact with the tube, which is essential for good boiling. Figure 4 in
Chapter 4 (Perry 1950; Stephan 1963a, 1963b, 1963c) of the 2005
ASHRAE Handbook—Fundamentals shows that the heat flux has a
dominant influence on the coefficient. For the range of temperatures
presented for R-22, the boiling coefficient drops by 12% as the boil-
ing temperature drops from –15°C to –41°C.
SECONDARY COOLANTS
Secondary coolant selection, system design considerations, and
applications are discussed in Chapter 4; properties of brines, inhib-
ited glycols, halocarbons, and nonaqueous fluids are given in Chap-
ter 21 of the 2005 ASHRAE Handbook—Fundamentals. The focus
here is on secondary coolants for low-temperature applications in
the range of –50 to –100°C.
An ideal secondary coolant should
• Have favorable thermophysical properties (high specific heat, low
viscosity, high density, and high thermal conductivity)
• Be nonflammable, nontoxic, environmentally acceptable, stable,
noncorrosive, and compatible with most engineering materials
• Possess a low vapor pressure
Only a few fluids meet these criteria, especially in the entire –50 to
–100°C range. Some of these fluids are hydrofluoroether (HFE),
diethylbenzene, d-limonene, polydimethylsiloxane, trichloroethyl-
ene, and methylene chloride. Table 9 provides an overview of these
coolants. Table 10 gives refrigerantproperties for the coolants at
various low temperatures.
Polydimethylsiloxane, known as silicone oil, is environmentally
friendly, nontoxic, and combustible and can operate in the whole
range. Because of its high viscosity (greater than 10 mPa·s), its flow
pattern is laminar at lower temperatures, which limits heat transfer.
d-Limonene is optically active terpene (C10H16) extracted from
orange and lemon oils. This fluid can be corrosive and is not recom-
mended for contact with some important materials (polyethylene,
polypropylene, natural rubber, neoprene, nitrile, silicone, and PVC).
Some problems with stability, such as increased viscosity with time,
are also reported. Contact with oxidizing agents should be avoided.
The values listed are based on data provided by the manufacturer in
a limited temperature range. d-Limonene is a combustible liquid
with a flash point of 46.1°C.
The synthetic aromatic heat transfer fluid group includes
diethylbenzene. Different proprietary versions of this coolant con-
tain different additives. In these fluids, the viscosity is not as strong
a function of temperature. Freezing takes place by crystallization,
similar to water.
Hydrofluoroether (1-methoxy-nonafluorobutane, C4F9CH3), is
a new fluid, so there is limited experience with its use. It is nonflam-
mable, nontoxic, and appropriate for the whole temperature range.
No ozone depletion is associated with its use, but its global warming
potential is 500 and its atmospheric lifetime is 4.1 years.
The alcohols (methanol and ethanol) have suitable low-tem-
perature physical properties, but they are flammable and methanol
is toxic, so their application is limited to industrial situations where
these characteristics can be accommodated.
Another possibility for a secondary coolant is acetone (C3H6O).
REFERENCES
Askeland, D.R. 1994. The science and engineering of materials, 3rd ed.
PWS Publishing, Boston. 
Dow Corning USA. 1993. Syltherm heat transfer fluids. Dow Corning Cor-
poration, Midland, MI.
Emhö, L.J. 1997. HC-recovery with low temperature refrigeration. Pre-
sented at ASHRAE Annual Meeting, Boston.
Enneking, J.C. and S. Priebe. 1993. Environmental application of Brayton
cycle heat pump at Savannah River Project. Meeting Customer Needs
with Heat Pumps, Conference/Equipment Show.
Florida Chemical Co. 1994. d-Limonene product and material safety data
sheets. Winter Haven, FL.
Hands, B.A. 1986. Cryogenic engineering. Academic Press, New York.
Jain, N.K. and Enneking, J.C. 1995. Optimization and operating experience
of an inert gas solvent recovery system. Air and Waste Management
Association Annual Meeting and Exhibition, San Antonio, June 18-23.
Perry, J.H. 1950. Chemical engineers handbook, 3rd ed. McGraw-Hill, New
York.
Raznjevic, K. 1997. Heat transfer. McGraw-Hill, New York.
Stephan, K. 1963a. The computation of heat transfer to boiling refrigerants.
Kältetechnik 15:231.
Stephan, K. 1963b. Influence of oil on heat transfer of boiling Freon-12 and
Freon-22. Eleventh International Congress of Refrigeration, I.I.R. Bulle-
tin No. 3.
Stephan, K. 1963c. A mechanism and picture of the processes involved in
heat transfer during bubble evaporation. Chemic. Ingenieur Technik
35:775.
Stoecker, W.F. and J.W. Jones. 1982. Refrigeration and air conditioning,
2nd ed. McGraw-Hill, New York.
Therminol LT. 1992. Technical Bulletin No. 9175. Monsanto, St. Louis.
3M Company. 1996. Performance Chemicals and Fluids Laboratory, St.
Paul, MN.
Weng, C. 1995. Non-CFC autocascade refrigeration system. U.S. Patent
5,408,848 (April).
BIBLIOGRAPHY
Wark, K. 1982. Thermodynamics, 4th ed. McGraw-Hill, New York.
Weng, C. 1990. Experimental study of evaporative heat transfer for a non-
azeotropic refrigerant blend at low temperature. M.A. thesis, Ohio
University.
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	Autocascade Systems 
	Custom-Designed and Field-Erected Systems 
	Single-Refrigerant Systems 
	Cascade Systems 
	Low-Temperature Materials 
	Insulation 
	Heat Transfer 
	Secondary Coolants 
	References 
	Bibliography 
	Tables
	 1 Low-Temperature Characteristics of Several Refrigerants at Three Evaporating Temperatures 
	 2 Properties of R-508b 
	 3 Theoretical Performance of a Cascade System Using R-13, R-503, R-23, or R-508b 
	 4 Theoretical Compressor Performance Data for Two Different Evaporating Temperatures 
	 5 Several Mechanical Properties of Aluminum Alloys at 196˚C 
	 6 Approximate Melting and Glass Transition Temperatures for Common Polymers 
	 7 Tensile Properties of Unidirectional Fiber-Reinforced Composites 
	 8 Components of a Low-Temperature Refrigerated Pipe Insulation System 
	 9 Overview of Some Secondary Coolants 
	 10 Refrigerant Properties of Some Low-Temperature Secondary Coolants 
	Figures
	 1 Simple Autocascade Refrigeration System 
	 2 Four-Stage Autocascade System 
	 3 Simple Cascade System 
	 4 Simple Cascade Pressure-Enthalpy Diagram 
	 5 Two-Stage Cascade System 
	 6 Three-Stage Cascade System 
	 7 Tensile Strength Versus Temperature of Several Metals 
	 8 Tensile Elongation Versus Temperature of Several Metals 
	 9 Shear Modulus Versus Normalized Temperature (T/Tg) for Thermoplastic Polymers 
	 10 Tensile Strength Versus Temperature of Plastics and Polymer Matrix Laminates